From 4bd6179f5211556874f1ca722b8a5c682a65f138 Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Mon, 9 Oct 2017 12:15:41 -0700
Subject: [PATCH] fscrypt: new helper function - fscrypt_prepare_link()

Introduce a helper function which prepares to link an inode into a
possibly-encrypted directory.  It handles setting up the target
directory's encryption key, then verifying that the link won't violate
the constraint that all files in an encrypted directory tree use the
same encryption policy.

Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
 fs/crypto/hooks.c               | 15 +++++++++++++++
 include/linux/fscrypt.h         | 27 +++++++++++++++++++++++++++
 include/linux/fscrypt_notsupp.h |  6 ++++++
 include/linux/fscrypt_supp.h    |  1 +
 4 files changed, 49 insertions(+)

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 069088e91ea9..8b90217320dd 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -47,3 +47,18 @@ int fscrypt_file_open(struct inode *inode, struct file *filp)
 	return err;
 }
 EXPORT_SYMBOL_GPL(fscrypt_file_open);
+
+int __fscrypt_prepare_link(struct inode *inode, struct inode *dir)
+{
+	int err;
+
+	err = fscrypt_require_key(dir);
+	if (err)
+		return err;
+
+	if (!fscrypt_has_permitted_context(dir, inode))
+		return -EPERM;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(__fscrypt_prepare_link);
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index cc86d49c820d..6ceb76abd842 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -178,4 +178,31 @@ static inline int fscrypt_require_key(struct inode *inode)
 	return 0;
 }
 
+/**
+ * fscrypt_prepare_link - prepare to link an inode into a possibly-encrypted directory
+ * @old_dentry: an existing dentry for the inode being linked
+ * @dir: the target directory
+ * @dentry: negative dentry for the target filename
+ *
+ * A new link can only be added to an encrypted directory if the directory's
+ * encryption key is available --- since otherwise we'd have no way to encrypt
+ * the filename.  Therefore, we first set up the directory's encryption key (if
+ * not already done) and return an error if it's unavailable.
+ *
+ * We also verify that the link will not violate the constraint that all files
+ * in an encrypted directory tree use the same encryption policy.
+ *
+ * Return: 0 on success, -ENOKEY if the directory's encryption key is missing,
+ * -EPERM if the link would result in an inconsistent encryption policy, or
+ * another -errno code.
+ */
+static inline int fscrypt_prepare_link(struct dentry *old_dentry,
+				       struct inode *dir,
+				       struct dentry *dentry)
+{
+	if (IS_ENCRYPTED(dir))
+		return __fscrypt_prepare_link(d_inode(old_dentry), dir);
+	return 0;
+}
+
 #endif	/* _LINUX_FSCRYPT_H */
diff --git a/include/linux/fscrypt_notsupp.h b/include/linux/fscrypt_notsupp.h
index ea26d4c609df..a80917bc2102 100644
--- a/include/linux/fscrypt_notsupp.h
+++ b/include/linux/fscrypt_notsupp.h
@@ -187,4 +187,10 @@ static inline int fscrypt_file_open(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+static inline int __fscrypt_prepare_link(struct inode *inode,
+					 struct inode *dir)
+{
+	return -EOPNOTSUPP;
+}
+
 #endif	/* _LINUX_FSCRYPT_NOTSUPP_H */
diff --git a/include/linux/fscrypt_supp.h b/include/linux/fscrypt_supp.h
index 0551352156e4..547cdbd04f17 100644
--- a/include/linux/fscrypt_supp.h
+++ b/include/linux/fscrypt_supp.h
@@ -146,5 +146,6 @@ extern int fscrypt_zeroout_range(const struct inode *, pgoff_t, sector_t,
 
 /* hooks.c */
 extern int fscrypt_file_open(struct inode *inode, struct file *filp);
+extern int __fscrypt_prepare_link(struct inode *inode, struct inode *dir);
 
 #endif	/* _LINUX_FSCRYPT_SUPP_H */